In the exploration and development of offshore petroleum reserves, it is often necessary to erect platforms located miles off shore. These platforms form a base on which drilling, exploration and storage activities can occur, and typically have legs or other types of support structure which extend down into the water. To transport men and material to and from these platforms, it is necessary to dock vessels alongside. In some situations, these vessels are small. In others, the vessels are quite large, and impact between these vessels and the platform leg structure can weaken or otherwise damage either the structure or the vessel itself.
To protect these platforms from damage due to contact by vessels operating near the platforms, bumper systems are attached to the platforms adjacent the water level and operate to fend off vessels and to absorb shocks from those vessels that come into contact with them.
These bumpers have found expression in a variety of constructions as exemplified by U.S. Pat. Nos. 3,991,582, 4,005,672, 4,098,211, 4,109,474 and 4,273,473. These basically include one or more surfaces for contact with the vessel or barge, and one or more shock-absorbing members interposed between the contact surfaces and the platform. The contact surfaces are chosen to provide a cushioning effect so as to spread the load on the hull surface over an area sufficient to prevent damage to the vessel. Thus, for example, one form of cushioning is provided by a plurality of resilient ring-like members that are disposed axially in a vertical configuration. A vertical pipe column maintains a stack of bumper rings on a common vertical axis and is supported top and bottom by one or more shock cells which absorb shocks. In some embodiments, a shock cell is provided at both top and bottom, while in others, a shock cell is installed at the top, and a resilient shear mounting is provided at the lower end.
Although the bumper systems (i.e., bumpers and shock cells) previously proposed have provided a marked improvement over arrangements having only bumpers, the provision of shock cells of suitable strength, resiliency, damping characteristic and cost have involved compromises due to the fact that they typically encounter vessels of a variety of sizes and shapes; and, consequently, the shock loads imparted to the bumpers/shock cells may vary widely. Moreover, even where vessels are of uniform size, varying sea conditions will result in greatly varying shock loads.
If a shock cell is designed to withstand the heavy loads resulting from large vessels in heavy seas, it may be insufficiently responsive to light loads imparted by small vessels and thereby cause damage to the small vessels. Correspondingly, if a shock cell is designed to have sufficient resiliency at low loads to properly cushion small vessels, it will be inadequate to provide the desired level of cushioning for larger vessels and thereby cause damage to the platform support. Accordingly, there has continued to be a need for a shock cell that incorporates good damping while providing levels of resiliency that accommodate a wide range of vessels under varying weather conditions.